The present invention generally relates to a brushless DC motor, and more particularly, one especially designed for use in driving a rotating hard disc in a hard disc drive, the motor having improved tooth saturation characteristics.
In a disc drive, data is stored on a disc supported for constant rotation by a spindle motor which typically comprises a brushless DC motor. The data accessed using an actuator which supports the transducer which was designed to fly over the surface of the disc as the disc rotates. In such permanent magnet motors, the small size of the motor, which is required for its use in a disc drive, makes optimization of all the motor parameters difficult. The motor typically comprises a stator having a plurality of teeth about which windings are placed to be energized to create the necessary rotation of the rotor. The rotor includes a thin rotor magnet which interacts with the flux generated at the stator to cause the necessary constant speed motor rotation. In such a small, high-powered motor, the air gap between the facing surfaces of the stator and the rotor must also be minimized. The combination of the minimal air gap, the thinness of all components, the slotted stator and the need for high performance, leads to a difficult design equation.
In such small, high performance motors, it is especially important to have a desired variation in the inductance of the motor, preferably by controlling the saturation as it changes with position of the teeth on the magnet. This produces several operational advantages. A problem in a motor of such small size is optimizing the saturation characteristics of the motor. Such optimized saturation can be difficult to achieve, because the saturation level changes as the stator teeth change position relative to the rotor magnets, and the ability to redistribute the flux path and density in the airgap between the stator teeth and the rotating magnet.
The magnetic saturation as a function of the position conditions the electromechanical devices"" electromagnetic performance in terms of back electromagnetic force (EMF) waveform and harmonics generated in the system. It is desirable to both optimize the saturation, and distribute or redistribute the flux density in the air gap to minimize torque ripple. The optimum redistribution of the flux density in the air gap will reduce the torque ripple and reduce acoustic noise. Thus an improved approach for optimizing tooth saturation and flux distribution patterns in the air gap is highly desired.
Therefore, it is an objective of the present invention to provide a permanent magnet excited motor having improved electromagnetic performance by improving the magnetic saturation level in the teeth of the stator.
It is a further objective of the invention to provide an improved motor with better distribution of the flux density in the air gap between the teeth of the stator and the rotor magnet.
A further objective of the invention is to provide a motor with an improved magnetic saturation level resulting in reduced electromagnetic harmonics to improve the motor performance.
Another objective of the invention is to provide an electromagnetic motor design with an improved distribution of flux density in the air gap to reduce the harmonics in the motor and thereby the acoustic noise.
Yet a further objective of the invention is to provide a motor especially for use as a spindle motor in a disc drive which has a design for the stator teeth which improves the magnetic saturation level of the teeth to allow for accurate switching of the motor windings.
A further objective of the invention is to provide a motor for use as a spindle motor in a disc drive in which the teeth are designed to redistribute the flux density in the air gap and reduce the harmonics and torque ripple and thereby acoustic noise performance of the motor.
In summary, the tooth saturation level is improved and electromagnetic harmonics reduced by reducing the path cross-section of area in the tooth with a geometrical means such as punched holes through the stator laminations. The punched holes alter the magnetic flux pattern by forcing the flux to go around the holes, providing a greater concentration of the flux within regions of the teeth, as well as, depending on the position of the hole along the long axis of the tooth, redistributing the flux density in the air gap. This redistributed flux density will give a specific desired form to the back EMF waveform. This controlled shaping of back EMF by controlling saturation will reduce the harmonics to improve the electromechanical device electromagnetic performance.
Other details and advantages of the present invention will become apparent to a person of skill in this field who studies the following figures in conjunction with the description of a preferred embodiment or embodiments to follow.